The April 2010 explosion at the Upper Big Branch (UBB) mine in West Virginia happened almost four years ago. A total of 29 miners died and two were seriously injured in a disaster that has forever scarred the lives of their families and loved ones. At UBB, a relatively small face ignition near the tailgate end of the longwall face turned into a major coal dust explosion that propagated through a large area of the mine with flames charring almost 50 miles of mine entries.1

Can such a devastating coal dust explosion happen again? Regrettably, yes, it can.

Every fall, the US Mine Safety and Health Administration (MSHA) issues a Winter Alert that warns of the dangers of coal dust explosions, which are more likely to occur during winter when the mine air is drier. Fine bituminous coal dust is highly explosive when suspended in air. In a typical coal dust explosion, the dust is scoured by a small methane gas explosion, which also provides the initiating flame. Coal dust explosions can also be triggered by blasting – especially from blown-out shots – and this often caused explosions in the early 20th Century.

Once initiated, a coal dust explosion can propagate through vast areas the mine by swirling new coal dust into the air in front of the explosion flame. This pattern will continue until there is no more coal dust left or until the explosion reaches an area where the coal dust has been sufficiently inertised by mixing it with rock dust. In writing his book on coal dust explosions, Cybulski conducted thousands of explosion tests in the Polish experimental mine “Barbara”.2 The following major findings were documented by Cybulski:

The finer the coal dust and the greater the coal’s volatile matter, the greater is its explosion hazard.

Coal dust explosions can be prevented by mixing the coal dust with inert dust (rock dust). If a large portion of the coal dust is finer than 200 mesh (74µm), more than 80% inert rock dust may be needed to prevent explosions.

If the initiating explosion is strong enough, even wet coal dust can explode.

Explosion barriers can stop coal dust explosions.

In the US, most mine operators rely on rock dust inertisation as the preferred method of protection against coal dust explosions. In 2011, the MSHA increased the required total inert content (TIC) for mine dust in intake airways from 65% to 80%, the same as it was (and is) for return airways. This change was based on the recognition from research by the National Institute for Occupational Safety and Health (NIOSH) that increasing mechanisation in today’s mines has also increased the fineness and therefore the risk of coal dust explosions. The transition to fully mechanised cutting and belt conveyor haulage produces more fine dust compared to old-style undercutting, blasting and track haulage.

To be effective in preventing explosions, the rock dust must be thoroughly mixed with the coal dust. If coal dust is permitted to form layers on top of rock-dusted surfaces, the explosion hazard increases because only the top 1/8 in. of dust is scoured up during an explosion. US Bureau of Mines and NIOSH tests have demonstrated that a coal dust layer only 4/1000 in. thick (equivalent to the thickness of a sheet of paper) can be sufficient to propagate a dust explosion. Therefore, the preferred method of applying rock dust is with trickle dusters that release rock dust directly into the return air, leaving the section, mixing it thoroughly with the coal dust particles in the air. Conveyor belts are generally batch dusted: here, frequent application of a light rock dusting is preferred over heavy dusting at longer intervals to prevent layering. Whether the amount of rock dust is sufficient to prevent coal dust explosions can be determined nearly instantly with the NIOSH-developed Coal Dust Explosibility Meter (CDEM).

European mine operators trap explosive coal dust with hygroscopic salts, including calcium and magnesium chloride solutions. The salts remain moist by attracting water from the mine air. The moist surface traps any coal dust settling on the treated surfaces. Salt applications last for several days, depending on the humidity of the mine air. Salts can be sprayed at any time because, unlike rock dust, they are not carried downwind where they obstruct visibility for the miners.

A second noteworthy technology for the prevention of coal dust explosions is the use of explosion barriers. Passive barriers consist of large troughs filled with water and suspended on shelves at strategic locations in the mine entries. The explosion pressure tips the shelves over and the water downpour quenches the flame, arresting the explosion. The UBB report by the West Virginia Office for Miners’ Health, Safety and Training notes that a pump sump in the UBB mine had evidently acted as an explosion barrier and prevented the explosion from further propagating into the headgate 21 entries.3

Roadheaders used for mine development in European mines are typically equipped with active, triggered barriers. If methane ignites near the cutter head, a sensor instantly triggers the release of fire extinguishing agent from six to eight pressurised containers mounted on the cutter boom to smother the flame. Since any minor face ignition may trigger a violent coal dust explosion, this active barrier technology is an important engineering control for the prevention of such explosions.

Conclusions

Coal dust continues to present a significant explosion hazard. Mine operators should follow a rigorous, comprehensive programme of rock dust application, maintenance and testing to ensure that sufficient levels of inertisation are present. More research should be conducted to determine which barrier technologies can be applied in US mines to provide an additional level of safety against frictional ignitions and in belt entries where rock dust maintenance is difficult.

Effective risk management in the coal industry is all about people. To succeed, you need to embed a risk-aware culture and make the risk process simple and relevant so that it becomes part of everyone’s daily job. Loren Padelford, Active Risk, US, explains further.